Magnetic field decay with Hall drift in neutron star crusts

TL;DR

This paper models the long-term decay of magnetic fields in neutron star crusts considering Hall drift, revealing complex nonlinear interactions and the evolution of magnetic field components over time.

Contribution

It provides a detailed analysis of magnetic field decay with Hall drift in neutron star crusts, including nonlinear coupling effects and field evolution dynamics.

Findings

Polar surface magnetic field becomes highly distorted during Hall drift.

Energy transfer favors the poloidal component when the toroidal dominates.

Surface dipole field eventually returns to initial configuration after long times.

Abstract

The dynamics of magnetic field decay with Hall drift is investigated. Assuming that axisymmetric magnetic fields are located in a spherical crust with uniform conductivity and electron number density, long-term evolution is calculated up to Ohmic dissipation. The nonlinear coupling between poloidal and toroidal components is explored in terms of their energies and helicity. Nonlinear oscillation by the drift in strongly magnetized regimes is clear only around the equipartition between two components. Significant energy is transferred to the poloidal component when the toroidal component initially dominates. However, the reverse is not true. Once the toroidal field is less dominant, it quickly decouples due to a larger damping rate. The polar field at the surface is highly distorted from the initial dipole during the Hall drift timescale, but returns to the initial dipole in a longer dissipation timescale, since it is the least damped one.